This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-90061, filed on Mar. 27, 2001; the entire contents of which are incorporated herein by reference.
The present invention generally relates to a diode. More specifically, the invention relates to a high-withstand-voltage, high-speed diode.
In general, a diode is used as a switching element in a rolling stock, such as an electric train. A diode of this type requires a high withstand voltage and a high speed operation.
FIG. 7 shows the structure of a cross section of a conventional high-withstand-voltage, high-speed diode, which is a part of diode structure shown in the U.S. Pat. No. 5,631,181.
Referring to FIG. 7, a p-type anode layer 102 is selectively diffused and formed in the central portion on the surface of a nxe2x88x92-type semiconductor substrate (semiconductor layer) 101, and an n-type stopper layer 104 is selectively diffused and formed in the peripheral edge portion of the layer 101 so as to surround the p-type anode layer 102. An anode electrode 103 is formed on the surface of the p-type anode layer 102 so as to ohmic-contact the surface of the p-type anode layer 102.
On the other hand, an n-type cathode layer 105 is diffused and formed on the whole reverse surface of the layer 101, and a cathode electrode 106 is formed on the surface of the n-type cathode layer 105 so as to ohmic-contact the surface of the n-type cathode layer 105.
In such a high-withstand-voltage, high-speed diode, if a forward voltage is applied between the anode electrode 103 and the cathode electrode 106, holes are injected into the layer 101 from the p-type anode layer 102, and electrons are injected into the layer 101 from the n-type cathode layer 105, so that the layer 101 is filled with high densities of holes and electrons to be in a high injection state. If the layer 101 is thus in a high injection state, a current flows between the anode and cathode electrodes at a very low voltage. Therefore, in order to decrease the forward voltage drop (ON-state voltage) of the high-withstand-voltage diode, it is an important design item to fill the layer 101 with pairs of electrons and holes (which will be hereinafter referred to as carriers).
On the other hand, during switching of the diode, it is required to release the carriers accumulated in the layer 101. As the number of the carriers accumulated in the layer 101 increases, it takes a lot of time to release the carrier, so that switching loss increases. Therefore, in order to improve switching loss, the layer 101 is generally designed to be thinner.
However, the smallest thickness for ensuring a reverse blocking voltage is physically fixed and can not be decreased, so that there is a problem in that switching loss is large.
According an embodiment of the present invention, there is provided a diode comprising:
a semiconductor layer of a first conductive type having a first principal plane and a second principal plane facing the first principal plane;
a first impurity layer of a second conductive type which is opposite to said first conductive type, said first impurity layer being selectively formed on said first principal plane of said semiconductor layer;
a second impurity layer of the first conductive type which is selectively formed on said first principal plane of said semiconductor layer apart from said first impurity layer;
a first main electrode connected to said first impurity layer;
a second main electrode connected to said second impurity layer;
a third impurity layer of the first conductive type which is selectively formed on said second principal plane of said semiconductor layer and which is formed so as to face said first impurity layer;
a fourth impurity layer of the second conductive type which is selectively formed on said second principal plane of said semiconductor layer and which is formed so as to face said second impurity layer; and
short-circuiting part to electrically connect said third impurity layer to said fourth impurity layer.
According to another embodiment of the present invention, there is provided a diode comprising:
a semiconductor layer of a first conductive type having a first principal plane and a second principal plane facing the first principal plane;
a first semiconductor layer of a second conductive type which is selectively formed on said first principal plane of said semiconductor layer;
a second impurity layer of the first conductive type which is selectively formed on said first principal plane of said semiconductor layer apart from said first impurity layer;
an electrical insulating region which is formed in said first semiconductor layer from said first principal plane of said semiconductor layer between said first and second impurity layers;
a first main electrode connected to said first impurity layer;
a second main electrode connected to said second impurity layer;
a third impurity layer of the first conductive type which is selectively formed on said second principal plane of said semiconductor layer and which is formed so as to face said first impurity layer;
a fourth impurity layer of the second conductive type which is selectively formed on said second principal plane of said semiconductor layer and which is formed so as to face said second impurity layer; and
short-circuit part to electrically connect said third impurity layer to said fourth impurity layer.
According to further embodiment of the present invention, there is provided a diode comprising:
a semiconductor layer of a first conductive type having a first principal plane and a second principal plane facing the first principal plane;
a first impurity layer of a second conductive type which is selectively formed on said first principal plane of said semiconductor layer;
a second impurity layer of the first conductive type which is selectively formed on said first principal plane of said semiconductor layer apart from said first impurity layer;
a low life time region which is formed in said semiconductor layer from said first principal plane of said semiconductor layer between said first and second impurity layers;
a first main electrode connected to said first impurity layer;
a second main electrode connected to said second impurity layer;
a third high-density impurity layer of the first conductive type which is selectively formed on said second principal plane of said semiconductor layer and which is formed so as to face said first impurity layer;
a fourth impurity layer of the second conductive type which is selectively formed on said second principal plane of said semiconductor layer and which is formed so as to face said second impurity layer; and
short-circuit part to electrically connect said third impurity layer to said fourth impurity layer.